1. Field of the Invention
The invention relates to positive dispersion optical fiber. More particularly, the invention relates to positive dispersion optical fiber having improved transmission characteristics including increased effective area.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
In an effort to improve the transmission capacity of optical fibers, wavelength division multiplexing (WDM) systems are used. In general, WDM systems multiplex a plurality of information channels onto a single fiber, with each channel operating at a different wavelength. To combat the inherent nonlinearity effects of interaction between channels (e.g., 4-photon mixing), many WDM system arrangements include a dispersion compensating arrangement including a positive dispersion fiber concatenated with a (negative) dispersion compensating fiber. The positive dispersion fiber typically comprises a single mode fiber with a small amount of dispersion to reduce the nonlinear interactions between channels. The dispersion compensating fiber tends to have a negative dispersion to reduce the introduced and otherwise accumulated dispersion.
However, dispersion compensating fibers tend to exhibit higher signal attenuation than non-dispersion compensating fibers. Thus, it is desirable for the positive dispersion fiber to have relatively low loss to reduce the overall attenuation of the dispersion compensating arrangement. Conventionally, low loss positive dispersion optical fibers exist. Such fibers include, e.g., the low loss, pure silica core fiber from Sumitomo Electric Industries, Ltd. See, e.g., xe2x80x9cUltra Low Nonlinearity Low Loss Pure Silica Core Fiber,xe2x80x9d Electronics Letters Online No: 19991094, Aug. 3, 1999.
However, it should be noted that the optical fibers disclosed in the above-referenced article have pure silica core regions, rather than more conventional optical fibers whose core regions are made of silica doped with, e.g., germanium dioxide (GeO2). Optical fibers having pure silica core regions typically are more expensive than GeO2-doped or other doped core fibers because, e.g., pure silica is more difficult and thus more expensive to process than Ge-doped silica. Also, pure silica core fibers have inherent difficulties associated with the depressed index cladding, e.g., the mismatch properties of the core and cladding make the fiber draw process more difficult.
Accordingly, it would be desirable to have an optical fiber, including a non-zero positive dispersion optical fiber having a relatively large effective area, that has the desirable transmission characteristics discussed hereinabove without the manufacturing and economic limitations of conventional pure silica core fiber.
The invention is embodied in an optical communications system including one or more optical transmission devices, one or more optical receiving devices, and at least one positive dispersion optical fiber coupled therebetween. Embodiments of the invention provide positive dispersion optical fiber that includes a doped core region with an index of refraction n1, a cladding region with an index of refraction n2, and first and second annular rings or regions with indices of refraction n3 and n4, respectively, formed between the doped core region and the cladding region. The various regions are manufactured in such a way that the refractive index value ranges are: 0.14 less than (n1xe2x88x92n2)/n2 less than 0.31, xe2x88x920.19 less than (n3xe2x88x92n2)/n2 less than xe2x88x920.02, and xe2x88x920.20 less than (n4xe2x88x92n2)/n2 less than xe2x88x920.08. The core region is doped, e.g., with germanium or other suitable material. The first and second regions are down-doped, e.g., with fluorine or other suitable material. The cladding region is, e.g., pure silica. Positive dispersion fiber according to embodiments of the invention has a chromatic dispersion greater than 20xc2x12.0 picosecond/(nanometer-kilometer) with a dispersion slope less than 0.062 ps/(nm2-km) at a wavelength of 1550 nm. Also, the mode field diameter (MFD) of the fiber is at least 11.9xc2x10.7 microns (xcexcm). Moreover, optical fiber according to embodiments of the invention has a relatively large effective core area, Aeff, e.g., greater than 100.0 xcexcm2, and a relative dispersion slope (RDS) less than 0.0032 nmxe2x88x92. Manufacture of the optical fiber includes manufacture of a core region with a width from approximately 6.0 xcexcm to approximately 6.4 xcexcm, a first annular region with a width from approximately 2.0 xcexcm to approximately 4.1 xcexcm, and a second annular region with a width from approximately 15.0 xcexcm to approximately 35.0 xcexcm. Optical fiber according to embodiments of the invention provides desired transmission characteristics such as relatively large effective core area and relatively low transmission loss at desired frequencies (e.g., 1550 nm) without being burdened by the manufacturing and economic limitations of conventional fibers.